1. Field of the Invention
The present invention relates to a wavelength-selective phase grating type optical low-pass filter, more particularly, to a wavelength-selective phase grating type optical low-pass filter comprising an adhesive layer between a pair of transparent layers, for use in an optical system such as a single-tube color video camera or a color video camera employing a solid-state image sensor having a color separation filter.
2. Description of the Prior Art
Conventionally, in a video camera or the like employing a solid-state CCD image sensor or an image pickup tube, an image of a subject focused on the CCD image sensor is spatially and discretely sampled to obtain output image data, with a predetermined sampling frequency equal to a Nyquist frequency of the CCD image sensor. In this case, when the optical image data of the subject image has spatial frequency components higher than the spatial sampling frequency of the CCD image sensor, the output image data generated by the solid-state CCD image sensor contains data of a configuration or phantom data which are not included in the subject data.
In other words, such frequency components higher than the Nyquist frequency which can not be picked up by the solid-state CCD image sensor are outputted in a form of phantom data such as aliasing components, Moire, and phantom color.
For this reason, in the above-mentioned video camera or the like, there has been conventionally employed a conventional practical method of limiting or attenuating such high spatial frequency components of a subject image data to be inputted to the solid-state CCD image sensor by arranging an optical low-pass filter in the CCD image sensor system, thereby preventing generation of phantom color and phantom signal.
For the above-mentioned purpose, the optical low-pass filter has been made utilizing a crystal plate having a birefringence characteristic or recently utilizing a phase grating. However, almost all the conventional optical low-pass filters have been intended for obtaining a low-pass effect generally equal over the whole wavelengths.
However, the following technique attempt has been put into practice taking advantage of the fact that the spatial distribution densities of respective color separation filters, i.e., the spatial frequencies thereof are different from each other in respective transmission wavelength bands. The technique is that an image signal in a wavelength band having a distribution of higher spatial frequencies is subjected to sampling at a relatively higher sampling frequency, while another image signal in a wavelength band having a distribution of lower spatial frequencies is subjected to sampling at a relatively lower sampling frequency, thereby obtaining appropriate image data.
A conventional optical low-pass filter comprising a pair of adjacent transparent layers adhering to each other has been suggested in U.S. Pat. No. 5,237,452, as issued, based on the Japanese patent laid-open publication No. 3-230156, wherein refractive indexes of adjacent transparent layers at a wavelength are the same as each other, and the adjacent transparent layers of materials have Abbe's numbers different from each other, thereby attenuating a particular wavelength component. However, when an adhesive layer between a pair of adjacent transparent layers has a relatively large thickness or a distribution of the thickness, the adhesive layer becomes a phase grating layer, and then there is a possibility of not obtaining a desirable low-pass effect.
When such a conventional optical low-pass filter is constructed using a pair of transparent layers made of materials different from each other, it is always necessary to insert an adhesive layer. In this case, there is such a possibility that the adhesive layer has a relatively large thickness or a distribution of the thickness. In this case, the adhesive layer becomes a phase grating layer, and then wavelength components may be attenuated or cut off up to a wavelength at which the low-pass effect is unnecessary.